Experiment Of Magnesium Alloy Solidification Structure Refinement Under High Pressure And First-principles Calculations Of Heterogeneous Nuclei

In this paper,under the 4GPa high pressure,the cast Mg-Al alloy was heated,melted,solidified by CS-1B-type high pressure six-sided top press,and high pressure solidified Mg-Al alloy compression experiments.The microstructure and mechanical properties of Mg-Al alloys were studied by means of compression tests and modern analytical methods such as SEM,XRD,and EBSD.The First-principle was used to calculate the lattice constants of a-Mg and β-Mg17A112 phases in Mg-Al alloys and the interfacial energy between MgY,Y phases and a-Mg in Mg-Zn-Y alloys.The dendritic growth inhibition factor(Q)and the mechanism of solidification microstructure of magnesium alloy under high pressure were discussed.The research indicated that:Compared with as-cast AZ91D/AZ31B,The solidification microstructure of Mg alloy under 4GPa were as follows:the number of nuclei per unit area increases;The average size of the grains decreases sharply;the β-Mg17Al12 by the as-cast into a granular or rod-shaped mass distribution in the matrix;dendrites more refined and equiaxed;and he solid solubility of A1 in the matrix increases.With the increase of the degree of Overheating,the grain size of the alloy gradually increases,and the number of nuclei decreases.It indicates that the Overheating is also one of the important factors that affect the characteristics of the solidified structure.The average hardness of AZ91D alloy reached 85HV under 4GPa high pressure,which was 11.6%higher than that of as-cast alloy,The compressive strength increased from as-cast 296MPa to 405Mpa;The compressive strength of AZ31B alloy at 4GPa-935 ℃ increases from 361MPa at casting to 407MPa at high pressure.The high-pressure solidification enhanced the mechanical properties of the alloy.The First-principles calculations show that the lattice constants of Mg and Mg17Al12 decrease at 4GPa.The mismatches between Mg and Mg17Al12 under high pressure are calculated by Bramfitt model.The mismatching degree are about 6%,It has been shown that high pressure coagulation can reduce the nucleation work and increase the nucleation substrate,the number of nuclei increased.According to the relationship between Q value and pressure:Q=0.4P3+0.6P2+22.6P+45.7,increasing the Q value suppresses the growth of dendrites.Therefore,the number of nuclei under high pressure,wettability improvement and Q value increase is the main mechanism of the alloy refinement.Calculate the interfacial energy of Mg/Y and MgY/Mg in Mg-Zn-Y alloy by the First-principle.The calculated results show that the interface energy of the two under 4GPa pressure is lower than that under atmospheric pressure.The smaller the interface energy is,the smaller the surface tension between the two is.The smaller the wetting angle θ,the stronger the nucleation ability of the heterogeneous interface.This is consistent with the experimental heterogeneous nucleation of Y and MgY which can be matrix in high pressure solidification process.